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Artificial intelligence and Internet of Things (IoT) technology, which are the core of the 4th industrial revolution, can resolve many problems that optimization of production times in the manufacturing process and reduction of materials required etc. In order to utilize the 4th industrial revolution technology, real-time monitoring technology of metal parts is essential, so technology for embedding sensors and IC chips into parts is essential. Using metal 3d printing technology, it is possible to embed IC chips into metal parts, which was impossible because of the existing high-temperature metal manufacturing process of casting or forging. Here we introduce a novel new method for sensor embedding into SUS316L by hemisphere design to avoid direct laser exposure onto sensors during selective laser melting process. Thermal and microstructural analysis was carried out to characterize the property of inner hemisphere for safe thermal couple embedding into SUS316L.

Process optimization of additively manufactured Ti–6Al–4V alloy is an important aspect of the production of engineered, high-performance parts for the aerospace and medical industries. In this study, the microstructural evolution and mechanical properties of direct energy deposition processed Ti–6Al–4V alloy were investigated using different processing parameters. Experimental analyses revealed that the line energy density corresponding to the processing parameters of the direct energy deposition process influences the properties of additively manufactured Ti–6Al–4V alloy. First, an optimal line energy density limits the incidence and size of voids resulting from a lack of fusion to enhance both alloy strength and ductility. Second, an excessively high energy density induces the coarsening of prior-β grains to impair both alloy strength with the Hall–Petch relationship and alloy ductility due to the plastic deformation instability caused by the limited number of grains. These results indicate that both the extent of fusion and prior-β grain size affect the mechanical properties of additively manufactured Ti–6Al–4V alloy. Moreover, the results demonstrate the utility of the line energy density-based approach in determining the optimal processing parameters for realizing high-performance materials.

The rapid melting and solidification cycle in additive manufacturing creates a non-equilibrium environment that induces metastable microstructures. These metastable microstructures include solute heterogeneity, dislocation cell structure and nano-sized precipitation, which contributes to the strength of additively manufactured alloys. Because the presence of metastable microstructure contributes to the mechanical property enhancement of additively manufactured alloy, quantification and estimation of strength by metastable microstructure becomes important issue. In this study, the role of dislocation cell structure on the mechanical property of additively manufactured stainless steels was investigated. The evolved cell networks not only interrupted dislocation gliding, but also acted as crack propagation paths during plastic deformation. The finer cell networks found in the additively manufacture 304L stainless steels induced more interactions with dislocations than those found in the additively manufacture 316L stainless steels, and that is related to the higher strength during tensile test. This result demonstrates the dislocation cell structure is a main strengthening mechanism for additively manufactured materials and the modified Hall–Petch hardening model successfully estimate the strengthening by cell boundaries.

Steel sheet is often shaped by straining to form a final product, and deformation process can influence the fatigue resistance. In this study, high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of precipitation hardened (PH) high strength steel with pre-straining levels of 0, 5 and 9% were investigated. Regardless of pre-straining level, fatigue cracks were found to be initiated at surface pits formed during pickling. The effect of pre-straining was not significant on the HCF resistance of PH steel, while the FCP rates increased with increasing level of pre-straining only in low and intermediate ΔK regimes. It was suggested that marginal increase on HCF resistance of PH steel was due to the combined effect of enhanced resistance to crack initiation and reduced resistance to crack propagation with pre-straining. The effect of pre-straining on HCF and FCP behavior of PH steel was discussed based on micrographic and fractographic observations.

The J-integral fracture toughness values of high-Mn steel (Fe-25 wt pct Mn) and 304L stainless steel were evaluated at 25 °C, 0 °C, − 50 °C, − 100 °C, − 163 °C, and − 196 °C using precracked compact tension (CT) specimens and compared to those determined using Charpy impact tests. The high-Mn steel exhibited excellent J-integral fracture toughness at both room and cryogenic temperatures, with values comparable to those of 304L stainless steel. However, the trend of the J-integral fracture toughness of high-Mn steel with the decreasing temperature differed from that of the Charpy impact test results. Electron backscattered diffraction and micrographic analyses suggest that the varying stacking fault energies of high-Mn steels at different temperatures affected the deformation behavior in the stretch zone at the crack tip of the CT specimen. The effect of this temperature-dependent deformation behavior of high-Mn steels on the fracture process in the J-integral test could differ from that in the Charpy impact test, resulting in the different trends in the fracture resistance with the decreasing temperature.

The high-cycle fatigue (HCF) behavior of dissimilar metal welds between high-Mn (HM) steel and 304L stainless steel was investigated at 298 K and 110 K. The resistance to HCF of the 25Mn/304L weld joint was comparable to that of a 304L/304L weld, even at 110 K. The HCF behavior of the dissimilar metal joints between HM steel and 304L could be reasonably predicted by the ultimate tensile strength at both room and cryogenic temperatures, as with the base metal. The failure locations and micro-hardness studies suggested that both geometrical and metallurgical factors were important in determining the HCF behavior of 25Mn/304L weld joints.

General design concept used in high-entropy alloys (HEAs) have deviated from forming an fcc single phase to utilizing hard intermetallic phases in ductile fcc matrix. Here, we effectively exploited strengthening effects of a brittle intermetallic sigma (σ) phase to improve cryogenic tensile properties of a non-equi-atomic ductile VCrFeNi four-component HEA. We preferentially selected vanadium as a candidate alloying element to efficiently produce the σ phase through computational thermodynamic approach. This σ phase has beneficial effects on grain refinement through retardation of grain growth due to grain-boundary pinning, thereby leading to yield strength of 0.79–0.93 GPa. The extensive strain hardening results in tensile strength of 1.33–1.49 GPa and ductility of 23–47% at cryogenic temperature, which are enabled by nano-sized dislocation substructures rather than deformation twinning. Our results demonstrate how the intermetallic σ phase, which has been avoided in typical HEAs because of ductility deterioration, could be used in high strength HEA design.

The fatigue crack propagation (FCP) behavior of β-annealed Ti-6Al-4V (Ti64) alloy was examined in air and 0.6 M NaCl solution under anodic and cathodic applied potentials and at two different R ratios of 0.1 and 0.7. β-annealed Ti64 alloy was sensitive to environmental FCP in NaCl solution under both anodic and cathodic applied potentials at an R ratio of 0.1, while the environmental effect was almost negligible at an R ratio of 0.7. The extent of crack branching in air and at an R ratio of 0.1 decreased substantially in NaCl solution and/or at an R ratio of 0.7. The EBSD (electron backscatter diffraction) and SEM (scanning electron microscope) fractographic analyses on the FCP tested specimens showed that microstructure-sensitive cracking, rather than crystallographic cleavage cracking, became encouraged in NaCl solution and/or high R ratio. It was suggested that the extent of crack branching played an important role in determining the environmental FCP behavior of β-annealed Ti64 alloy.

The effects of cooling rate on the stress corrosion cracking (SCC) susceptibility of β-processed Ti–6Al–4V (Ti64) alloy, including BA/S specimen with furnace cooling and BQ/S specimen with water quenching, were investigated in 0.6M NaCl solution under various applied potentials using a slow strain rate test technique. It was found that the SCC susceptibility of β-processed Ti64 alloy in aqueous NaCl solution decreased with fast cooling rate, which was particularly substantial under an anodic applied potential. The micrographic and fractographic analyses suggested that the enhancement with fast cooling rate was related to the random orientation of acicular α platelets in BQ/S specimen. Based on the experimental results, the effect of cooling rate on the SCC behavior of β-processed Ti64 alloy in aqueous NaCl solution was discussed.

A variety of factors affect the fatigue behavior of high-Mn steels, which include both extrinsic (i.e., loading type, R ratio, specimen type, surface condition, temperature, and environment) and intrinsic (i.e., chemical composition, grain size, microstructure, stacking fault energy) factors. Very often, the influence of extrinsic factors on the fatigue behavior is even greater than that of intrinsic factors, misleading the interpretation of fatigue data. The metallurgical factors influence the initiation and propagation behaviors of fatigue by altering the characteristics of slip that is prerequisite for fatigue damage accumulation. It is however not easy to separate the effect of each factor since they affect the fatigue behavior of high-Mn steels in complex and synergistic way. In this review, the fatigue data of high-Mn steels are summarized and the factors complicating the interpretation are discussed.

The effect of stabilization annealing on the stress corrosion cracking (SCC) susceptibility of β-annealed Ti-6Al-4V (Ti64) alloy was examined in an aqueous 0.6 M NaCl solution under various applied potentials of +0.1, -0.05 and -0.1 V vs Ecorr, respectively, at a strain rate of 10 -6 s -1. The stabilization annealing substantially improved the resistance to SCC of β-annealed Ti64 alloy in 0.6 M NaCl solution under cathodic applied potentials, while the effect was marginal under an anodic applied potential. It was also noted that the areal fraction between ductile and brittle fracture of β-annealed Ti64 specimens, which were slow strain rate tested in 0.6 M NaCl solution, varied with stabilization annealing and applied potentials. The effect of stabilization annealing on the SCC behavior of β-annealed Ti64 alloy in SCC-causing environment was discussed based on the micrographic and fractographic observation.

The passive films on typical stainless steels (SS) and on a newly developed high-Cr (17 wt%)-added 18 wt%-Mn steel (HCr-HMnS) were compared by Cs-corrected scanning transmission electron microscopy and atom probe tomography. Although the passive films of all samples having similar Cr contents had the same thickness, unprecedented hexagonal wurtzite MnO inside the passive film of HCr-HMnS specimen was susceptible to corrosion cracking; this was not observed in the SS samples. This MnO caused crack formation during potentiodynamic polarization test, suggesting that reducing the harmful MnO by adding Mo and Ni facilitates the development of high-Mn base SS materials . Furthermore, higher MoO2 composition of the passive films on 316 type austenitic SS than 304 type series might would result in primarily the improved pitting resistance.

Diffused hydrogen atoms in high-strength steels induce both cohesive energy drops and dislocation mobility enhancement, which usually degrade the mechanical properties of materials. However, the enhanced dislocation mobility could also increase the slip planarity, which enhances the mechanical properties of materials. In this study, effects of hydrogen charging on the mechanical properties of as-cast high-Mn steels were investigated. Hydrogen charging in the high-Mn steel promotes slip planarity with short-range ordering, which results in a large planar slip band fraction and strain hardening enhancement of the hydrogen-charged sample. This shows that hydrogen-induced ordering can be related to both deformation mechanism and mechanical property of hydrogen charged high-Mn steels.

The effects of stabilization annealing and cooling rate on high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of β-processed Ti64 alloys were examined. After β-process heating above β transus, two different cooling rates of air cooling (β-annealing) and water quenching (β-quenching) were utilized. Selected specimens were then underwent stabilization annealing. The tensile tests, HCF and FCP tests on conducted on the β-processed Ti64 specimens with and without stabilization annealing. No notable microstructural and mechanical changes with stabilization annealing was observed for the β-annealed Ti64 alloys. However, significant effect of stabilization annealing was found on the FCP behavior of β-quenched Ti64 alloys, which appeared to be related to the built-up of residual stress after quenching. The mechanical behavior of β-processed Ti64 alloys with and with stabilization annealing was discussed based on the micrographic examination, including crack growth path and crack nucleation site, and fractographic analysis.

Four types of strain-based API X60 pipeline steels were fabricated by varying the C and Si contents, and the effects of C and Si on strain aging were investigated. The 0.05 wt% C steels consisted mainly of polygonal ferrite (PF), whereas the 0.08 wt% C steels consisted of acicular ferrite (AF). The volume fraction of AF increased with increasing C content because C is an austenite stabilizer element. The volume fractions of bainitic ferrite (BF) of the 0.15 wt% Si steels were higher than those of the 0.25 wt% Si steels, whereas the volume fractions of the secondary phases were lower. From the tensile properties before and after the aging process of the strainbased API X60 pipeline steels, the yield strength increased and the uniform and total elongation decreased, which is the strain aging effect. The strain aging effect in the strain-based API X60 pipeline steels was minimized when the volume fraction of AF was increased and secondary phases were distributed uniformly. On the other hand, an excessively high C content formed fine precipitates, and the strain aging effect occurred because of the interactions among dislocations and fine precipitates.

Tensile and high cycle fatigue behaviors of high-Mn austenitic steels, including 25Mn, 25Mn0.2Al, 25Mn0.5Cu, 24Mn4Cr, 22Mn3Cr and 16Mn2Al specimens, were investigated at 298 and 110 K. Depending on the alloying elements, tensile ductility of high-Mn steels either increased or decreased with decreasing temperature from 298 to 110 K. Reasonable correlation between the tendency for martensitic tranformation, the critical twinning stress and the percent change in tensile elongation suggested that tensile deformation of high-Mn steels was strongly influenced by SFE determining TRIP and TWIP effects. Tensile strength was the most important parameter in determining the resistance to high cycle fatigue of high-Mn steels with an exceptional work hardening capability at room and cryogenic temperatures. The fatigue crack nucleation mechanism in high-Mn steels did not vary with decreasing tempertature, except Cr-added specimens with grain boundary cracking at 298 K and slip band cracking at 110 K. The EBSD (electron backscatter diffraction) analyses suggested that the deformation mechanism under fatigue loading was significantly different from tensile deformation which could be affected by TRIP and TWIP effects.

The large strength difference between hard and soft components in heterogeneous structured materials leads to the evolution of high back-stress hardening, which increases the strength and ductility of materials simultaneously. Moreover, the combination of high shear strain and elevated temperature allows an increase in the strength of low-melting temperature metallic alloys by grain refinement, solute migration, and clustering. In this study, to design a new heterogeneous microstructure in the aluminum alloy, both room temperature (RT) and high-temperature (HT) ultrasonic nanocrystalline surface modification (UNSM) were conducted, and their mechanical properties and microstructural evolutions were investigated. The large shear strain from the UNSM treatment reduces the grain size at the sample surface and creates a gradient structure. The combination of shear strain and elevated temperature during UNSM treatment induces solute migration at a certain depth of the specimens, resulting in the nano-sized Mg-rich particles at the surface region. Both grain refinement and precipitation at the surface region of the HT sample provide strong back-stress hardening in the early stages of deformation that enhances the strength and ductility of materials. Therefore, a high shear strain and control of processing temperature allow the design of a unique heterogeneous microstructure in low-melting temperature metallic alloys, which is a good strategy for enhancing the mechanical properties of sheet or thin metallic products.

High frictional die and continuous rotation during high-pressure torsion provide not only a large shear strain but also a large amount of frictional heat that facilitates the generation of complex microstructural changes, including grain refinement, dynamic recovery, and solute migration. Since metallic alloys with low melting temperatures are sensitive to heat energy, the mechanical properties and microstructural evolution of high-pressure torsion-processed aluminum 7075 alloys are investigated in this study. The large shear strain resulting from high-pressure torsion induces both grain refinement and dislocation cell formation, leading to strength enhancement and ductility degradation of the alloy. The deformation and frictional heat become a driving force for solute migration, including brittle intermetallic compound dissolution and nano-sized precipitation generation in the matrix. These solute migration activities contribute toward enhancing both the strength and ductility by inducing precipitation hardening and dissolution of the brittle phase simultaneously. Consequently, both the strength and ductility of the high-pressure torsion-processed aluminum 7075 alloy become larger than that of the initial state. This result shows that the microstructural changes of the severe plastic deformation-processed low-melting-temperature metallic alloys induce a significant mechanical property enhancement of nanocrystalline materials.

Despite recent progress in achieving high mechanical properties of 3D printed metal products, the low productivity still remains a major limitation for their cost-effective feasibility in practical applications. To achieve high-speed printing with affordable mechanical properties, we increased the scanning speed of selective laser melting process with Ti–6Al–4V up to 1800 mm/s and applied a hot isostatic pressing (HIP) process to compensate for the porosity. In these high-speed printed specimens, the HIP process led to a microstructural change from αʹ-lath martensite to a Widmanstӓtten α-lamellar structure, which deteriorated their tensile properties due to the segregation of β-stabilizing atoms and caused inter-lamellar fracture. The deterioration phenomenon of high-speed printed Ti–6Al–4V specimens after the HIP process was found to be critically affected by the surface roughness of as-built state, which can be efficiently controlled with a build angle set-up.

A single-phase face-centered-cubic (FCC) high- or medium-entropy alloys (HEAs or MEAs) have attracted great attentions due to their novel damage-tolerance properties (strength, ductility, and fracture toughness) by generating nano-twins at cryogenic temperature. The fracture toughness assessment is essential for evaluating the reliability of high-performance materials for cryogenic applications; however, fracture studies on single-phase FCC HEAs showing transformation-induced plasticity (TRIP) have been hardly conducted. In this study, thus, damage-tolerance mechanisms of a V10Cr10Fe45Co30Ni5 HEA showing the FCC to body-centered-cubic (BCC) TRIP were investigated at room and cryogenic temperatures. At room temperature (298 K), the alloy shows the tensile strength of 731 MPa, elongation of 40%, and fracture toughness (KJIc) of 230 MPa m1/2. At cryogenic temperature (77 K), the strength and elongation improve to 1.2 GPa and 66%, respectively, while the KJIc remains almost constant at 237 MPa m1/2. Dislocation-mediated plasticity prevails at 298 K; however, the TRIP from FCC to BCC occurs at 77 K. Deformation and fracture mechanisms are analyzed by stacking fault energies and differences in Gibbs free energies between phases calculated by ab-initio methods, and are compared to those of CrMnFeCoNi, CrCoNi, Fe50Mn30Co10Cr10, and V10Cr10Fe45Co20Ni15 alloys. Despite the presence of a considerable amount of BCC which is intrinsically brittle at low temperature, the transformed BCC martensite shows ductile fracture after the fracture toughness test even in cryogenic environments. These results demonstrate that the FCC to BCC TRIP can be an attractive route in a field of HEA design to overcome the strength and toughness trade-off at cryogenic temperature.

High-Mn austenitic steels utilizing TWIP (twinning induced plasticity) effect have excellent combination of tensile strength and ductility. The near-threshold fatigue crack propagation (FCP) behavior, as represented by the ΔK th value, of high-Mn steels was examined with the emphasis on the effect of stacking fault energy (SFE), grain size, twinning and tensile properties. Even though no predominant parameter determining the near-threshold FCP behavior of high-Mn steels was found, the SFE showed the most reasonable correlation to the ΔK th values among the variables examined. It was also suggested that the slip reversibility as determined by SFE could not solely explain the near-threshold FCP behavior of high-Mn steels. The presence of twin boundaries appeared to be not beneficial in improving the resistance to FCP of high-Mn steels in low ΔK regime. The near-threshold FCP characteristics of high-Mn austenitic steels were discussed and correlated with the well-known parameters presumably affecting FCP.

The S-N fatigue behavior of newly developed Fe25Mn steel, including base metal and butt-welded joint, was investigated at 298 and 110 K, and the results were compared to those of previously reported Fe16Mn2Al and STS304L steels. Fe25Mn steel has quite promising fatigue performance at 298 K and even at 110 K, showing comparable resistance to fatigue to STS304L. The S-N fatigue behavior of Fe25Mn steel was dependent on tensile strength at 298 and 110 K, the trend of which well agreed to that of other austenitic steels. The electron backscatter diffraction and micrographic analyses suggested that transformation induced plasticity and twinning induced plasticity effects did not occur in Fe25Mn steel under fatigue loading at room and cryogenic temperatures. The butt-welded Fe25Mn/Fe25Mn and Fe25Mn/STS304L specimens also showed a satisfactory fatigue behavior which was even comparable to that of STS304L/STS304L specimen at 110 K. The S-N fatigue behavior of Fe25Mn steel and its welds was discussed based on the fractographic and microscopic observations.

The fatigue crack propagation (FCP) behavior of Fe25Mn and Fe16Mn2Al austenitic steels was investigated at 298 and 110 K, and the results were compared with the reported results of Fe24Mn2Cr steel. It was found that the FCP behavior of high-Mn, austenitic steels was largely influenced by the stacking fault energy (SFE) and the grain size. The resistance to FCP of high-Mn steels in this study was enhanced in the near-threshold ΔK regime with decreasing temperature from 298 to 110 K. The improvement for the Fe25Mn and the Fe16Mn2Al specimen was, however, marginal as compared to that of the Fe24Mn2Cr specimen. Other than the change in SFE, the secondary cracking at cryogenic temperature appeared to affect the FCP behavior of high-Mn steels, since the secondary cracks perpendicular to the crack propagating direction could reduce the effective stress intensity factor, decreasing the FCP rates. Sufficiently high stress concentration at grain boundary tended to occur at low temperature for relatively large grain sized Fe24Mn2Cr specimen and cause the secondary cracking, but not for the Fe25Mn and the Fe16Mn2Al specimen.

The effect of superplastic forming (SPF) on tensile and S (stress)-N (number of cycles to failure) fatigue properties of Ti64 alloy was examined at 298 and 473 K. For simulating the superplastic forming exposure, millannealed Ti64 alloy sheet was heated in a vacuum chamber with a pre-determined temperature profile. For some as-exposed specimens, the α-case formed on the surface during expousre was mechanically removed to understand the effect of α-case on the mechanical properties of Ti64 alloy. It was found that the presence of α-case significantly affected the tensile and the fatigue properties of Ti64 alloy at 298 and 473 K by providing an easy initiation site for both tensile and fatigue fracture. The microstructural change during the SPF exposure was marginal in affecting the S-N fatigue properties of Ti64 alloy. Different testing temperature of 298 and 473 K affected the S-N fatigue behavior of as-received and as-exposed (α-case removed) Ti64 specimens, but not that of as-exposed specimen.

The correlation between the microstructures and tensile properties of strain-based American Petroleum Institute (API) X60 pipeline steels was investigated. Eight types of strain-based API X60 pipeline steels were fabricated by varying the chemical compositions, such as C, Ni, Cr, and Mo, and the finish cooling temperatures, such as single-phase and dual-phase regions. In the 4N and 5C steels, the volume fractions of bainitic ferrite (BF) and the secondary phases increased with the increasing C and adding Cr instead of Ni. In the 5C and 6NC steels, the volume fractions of acicular ferrite (AF) and BF decreased with increasing C and adding Ni, whereas the volume fractions of polygonal ferrite (PF) and the secondary phases increased. In the 6NC and 6NM steels, the volume fraction of BF was increased by adding Mo instead of Cr, whereas the volume fractions of PF and the secondary phases decreased. In the steels rolled in the single-phase region, the volume fraction of polygonal ferrite ranged from 40 to 60 pct and the volume fraction of AF ranged from 20 to 40 pct. In the steels rolled in the dual-phase region, however, the volume fraction of PF was more than 70 pct and the volume fraction of AF was below 20 pct. The strength of the steels with a high volume fraction of AF was higher than those of the steels with a high volume fraction of PF, whereas the yield point elongation and the strain hardening exponent were opposite. The uniform elongation after the thermal aging process decreased with increasing volume fraction of PF, whereas the uniform elongation increased with increasing volume fraction of AF. The strain hardening exponent increased with increasing volume fraction of PF, but decreased with increasing volume fraction of AF and effective grain size.

Two types of strain-based American Petroleum Institute (API) X60 linepipe steels were fabricated at two finish cooling temperatures, 673 K and 723 K (400 °C and 450 °C), and the effects of the finish cooling temperatures on the tensile properties after thermal aging were investigated. The strain-based API X60 linepipe steels consisted mainly of polygonal ferrite (PF) or quasi-polygonal ferrite and the volume fraction of acicular ferrite increased with the increasing finish cooling temperature. In contrast, the volume fractions of bainitic ferrite (BF) and secondary phases decreased. The tensile properties before and after thermal aging at 473 K and 523 K (200 °C and 250 °C) were measured. The yield strength, ultimate tensile strength, and yield ratio increased with the increasing thermal aging temperature. The strain hardening rate in the steel fabricated at the higher finish cooling temperature decreased rapidly after thermal aging, probably due to the Cottrell atmosphere, whereas the strain hardening rate in the steel fabricated at the lower finish cooling temperature changed slightly after thermal aging. The uniform elongation and total elongation decreased with increasing thermal aging temperature, probably due to the interactions between carbon atoms and dislocations. The uniform elongation decreased rapidly with the decreasing volume fractions of BF and martensite and secondary phases. The yield ratio increased with the increasing thermal aging temperature, whereas the strain hardening exponent decreased. The strain hardening exponent of PL steel decreased rapidly after thermal aging because of the large number of mobile dislocations between PF and BF or martensite or secondary phases.

The micromechanics of plastic deformation and phase transformation in a three-phase advanced high strength steel are analyzed both experimentally and by microstructure-based simulations. The steel examined is a three-phase (ferrite, martensite and retained austenite) quenched and partitioned sheet steel with a tensile strength of ~980 MPa. The macroscopic flow behavior and the volume fraction of martensite resulting from the austenite–martensite transformation during deformation were measured. In addition, micropillar compression specimens were extracted from the individual ferrite grains and the martensite particles, and using a flat-punch nanoindenter, stress–strain curves were obtained. Finite element simulations idealize the microstructure as a composite that contains ferrite, martensite and retained austenite. All three phases are discretely modeled using appropriate crystal plasticity based constitutive relations. Material parameters for ferrite and martensite are determined by fitting numerical predictions to the micropillar data. The constitutive relation for retained austenite takes into account contributions to the strain rate from the austenite–martensite transformation, as well as slip in both the untransformed austenite and product martensite. Parameters for the retained austenite are then determined by fitting the predicted flow stress and transformed austenite volume fraction in a 3D microstructure to experimental measurements. Simulations are used to probe the role of the retained austenite in controlling the strain hardening behavior as well as internal stress and strain distributions in the microstructure.

Six low-carbon high-strength bainitic steels containing Cr, Mo, and B were fabricated by controlling finish cooling temperature, and the effect of bainitic microstructure on tensile and Charpy impact properties were investigated. All the specimens were composed primarily of bainitic ferrite, together with small amounts of granular bainite, acicular ferrite, martensite–austenite constituent. These bainitic microstructures were more critically affected by the finish cooling temperature than by the alloying elements. The H-series specimens with a high finish cooling temperature had larger amount of acicular ferrite and smaller amount of granular bainite and bainitic ferrite, compared to the L-series specimens with the low finish cooling temperature at the same chemical composition. The L-series specimens exhibited higher strength and yield ratio, and lower uniform and total elongations than the H-series specimens because the volume fraction of BF was higher in the L-series specimens than in the H-series specimens. On the other hand, the energy transition temperature decreased with increasing the volume fraction of AF having fine effective grain size, while it increased with an increase in the volume fraction of GB having coarse effective grain size. Thus, the energy transition temperature of the H-series specimens with the high finish cooling temperature were slightly lower than that of the L-series specimens with the low finish cooling temperature because the H-series specimens had a larger amount of AF than the L-series specimens.

Representative face-centered-cubic (FCC) high-entropy alloys (HEAs) or medium-entropy alloys (MEAs), e.g., equi-atomic CoCrFeMnNi or CrCoNi alloys, have drawn many attentions due to the excellent damage-tolerance at cryogenic temperature. The investigation of fracture toughness at 77 K is basically required for the reliable evaluation of high-performance alloys used for cryogenic applications; however, it has been rarely carried out for the non-equi-atomic FCC HEAs yet. In this study, tensile and fracture toughness tests were conducted on the non-equi-atomic V10Cr10Fe45Co20Ni15 alloy, and the results were compared with those of the equi-atomic CoCrFeMnNi and CrCoNi alloys. The present alloy shows a good damage tolerance at cryogenic temperature with tensile strength of 1 GPa and elongation of ∼60%. The KJIc fracture toughness values are 219 and 232 MPa m1/2 at 298 and 77 K, respectively, showing the increase in toughness with decreasing temperature. This increase results from the absence of twins at 298 K and the increased propensity to twin formation at 77 K, which is well confirmed by the variation of stacking fault energies (SFEs) by using Ab-initio calculations. The mechanical properties of the present alloy are actually similar or slightly lower than those of the other CoNiCr or FeMnCoNiCr alloy; instead, this study provides that neither composition nor certain elements are the most important factors dictating damage-tolerance of HEAs or MEAs.

The microstructural-mechanical correlative study has been conducted for characterization of selective laser melted H13 tool steel. Transformation behavior from austenite to martensite has been observed with partitioning of C in matrix with correlative atomic diffusivity during selective laser melting process. During solidification, columnar grain structures are formed due to epitaxial growth following the build direction of H13 tool steels. Columnar microstructures are mostly composed of martensite with small amount of retained austenite. Supercooling of H13 with high laser scan speed increased the nucleation sites, which reduced the diameter of columnar grain. During tensile test, deformation appeared in grain boundary while there was no significant martensitic phase transformation confirmed by X-ray diffraction (XRD) method and electron backscattered diffraction (EBSD) analysis. S2 (scan speed of 200 mm/s specimen had the better tensile property with tensile strength of 1700 MPa and elongation of 1.6% than the rest(

The effects of start and finish cooling temperatures on microstructure and mechanical properties of low-carbon high-strength and low-yield ratio bainitic steels were investigated in this study. Four kinds of low-carbon high-strength and low-yield ratio bainitic steels were fabricated by varying the start and finish cooling temperatures and cooling rates, and their microstructure and mechanical properties such as tensile and Charpy impact properties were measured. In the steels cooled down from the high start cooling temperature above Ar1 [978 K (705 °C)], the volume fraction of acicular ferrite is lower than in the steels cooled down from the low start cooling temperature below Ar1 [978 K (705 °C)]. The finish cooling temperatures and cooling rates affect the formation of bainitic ferrite, granular bainite, and martensite–austenite (MA) constituents. According to the correlation between microstructure and mechanical properties, the tensile strength increases with increasing the volume fractions of bainitic ferrite and MA constituents, whereas the elongation decreases. The yield ratio decreases as the volume fraction of MA constituents increases. Charpy impact absorbed energy is proportional to the volume fraction of acicular ferrite, and is inversely proportional to the volume fraction of granular bainite.

Two-ply Al/Cu sheets were prepared via roll bonding with different reduction ratios. Al/Cu sheets fabricated below 50% of reduction ratio exhibited relatively equiaxed grains without interface reaction, which resulted in weak joint-bonding strength. However, both strong metallurgical bonding at interface and fine, elongated grains from constituent alloys adjacent to the interface were successfully introduced under the reduction ratio of 65%, leading to a strongly enhanced bonding strength of 17.1 N/mm together with an increased elongation up to fracture by 28%.

In this study, four low-carbon high-strength bainitic steel specimens were fabricated by varying finish cooling temperatures and cooling rates, and their tensile and Charpy impact properties were investigated. All the bainitic steel specimens consisted of acicular ferrite, granular bainite, bainitic ferrite, and martensite-austenite constituents. The specimens fabricated with higher finish cooling temperature had a lower volume fraction of martensite-austenite constituent than the specimens fabricated with lower finish cooling temperature. The fast-cooled specimens had twice the volume fraction of bainitic ferrite and consequently higher yield and tensile strengths than the slow-cooled specimens. The energy transition temperature tended to increase with increasing effective grain size or with increasing volume fraction of granular bainite. The fast-cooled specimen fabricated with high finish cooling temperature and fast cooling rate showed the lowest energy transition temperature among the four specimens because of the lowest content of coarse granular bainite. These findings indicated that Charpy impact properties as well as strength could be improved by suppressing the formation of granular bainite, despite the presence of some hard microstructural constituents such as bainitic ferrite and martensite-austenite.

The effects of B and Cu addition and cooling rate on microstructure and mechanical properties of low-carbon, high-strength bainitic steels were investigated in this study. The steel specimens were composed mostly of bainitic ferrite, together with small amounts of acicular ferrite, granular bainite, and martensite. The yield and tensile strengths of all the specimens were higher than 1000 MPa and 1150 MPa, respectively, whereas the upper shelf energy was higher than 160 J and energy transition temperature was lower than 208 K (–65 °C) in most specimens. The slow-cooled specimens tended to have the lower strengths, higher elongation, and lower energy transition temperature than the fast-cooled specimens. The Charpy notch toughness was improved with increasing volume fraction of acicular ferrite because acicular ferrites favorably worked for Charpy notch toughness even when other low-toughness microstructures such as bainitic ferrite and martensite were mixed together. To develop high-strength bainitic steels with an excellent combination of strength and toughness, the formation of bainitic microstructures mixed with acicular ferrite was needed, and the formation of granular bainite was prevented.

In this study, drop-weight tear tests (DWTT) were conducted on API X80 pipeline steels fabricated with different finish rolling temperatures in order to analyze abnormal fracture appearance, i.e ., inverse fracture, occurring in the region impacted by a hammer. Area fractions of fracture modes were measured from fractured DWTT specimens, and the measured data were analyzed in relation to microstructures, DWTT absorbed energy, and strain hardening of the hammer-impacted region. As the finish rolling temperature decreased, the volume fraction of fine-grained acicular ferrite increased, while that of large-grained upper bainite or granular bainite decreased. According to the DWTT results, the absorbed energy tended to increase with increasing volume fraction of acicular ferrite (with decreasing finish rolling temperature). A large area of inverse fracture of cleavage type was found in the hammer-impacted region of the steels fabricated with high finish rolling temperatures, but the area fraction of inverse fracture was reduced in the steels fabricated with low finish rolling temperatures. Since the area fraction of inverse fracture was closely related with strain hardening of the hammer-impacted region, it could be successfully reduced by lowering strain hardening and by promoting the formation of acicular ferrite via low finish rolling temperatures.

The effects of carbon equivalent and cooling rateon tensile and Charpy impact properties of high-strength bainitic steels were investigated. Eight steel plates were fabricated with varying C, Cr, and Nb additions under two different cooling rates, and their microstructures, tensile, and Charpy impact properties were evaluated. Volume fractions of microstructural components present in the steels increased in the order of granular bainite, acicular ferrite, bainitic ferrite, and martensite as the carbon equivalent or cooling rate increased, which resulted in decreased ductility and upper shelf energy and increased energy transition temperature in spite of increased strength. In the steels containing about 50 vol.% of bainitic ferrite and martensite, the tensile strength was about 900 MPa, while the elongation and upper shelf energy were about 20% and 200 J, respectively. In order to achieve the best combination of tensile strength, ductility, and upper shelf energy, e.g., 860–900 MPa, 20%, and 200 J, respectively, granular bainite, and acicular ferrite were produced by controlling the carbon equivalent and cooling rate, while about 50 vol.% of bainitic ferrite and martensite were maintained to keep the high strength.

This study was concerned with effects of acicular ferrite on Charpy impact properties in heat affected zones (HAZs) of two API X80 linepipe steels containing oxides. In the one steel, Mg and O 2 were additionally added to form a larger amount of oxides than the other steel, which was a conventional X80 steel containing a considerable amount of Al and Ti. Various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs of 35 kJ cm −1 and 60 kJ cm −1 . Oxides present in the API X80 linepipe steels were complex oxides whose average size was 1–2 μm, and the number of oxides increased with increasing amount of Mg and O 2 . The volume fraction of acicular ferrite present in the steel HAZs increased with increasing number of oxides, and decreased with increasing heat input. When the volume fraction of acicular in the HAZ was higher than 20%, Charpy impact energy at −20 °C was higher than 100 J as the ductile fracture mode was dominant. Particularly in the steel HAZs having a larger amount of oxides, Charpy impact properties were excellent because oxides worked as nucleation sites of acicular ferrite during welding. Charpy impact properties of the HAZs could be well correlated with the volume fraction of acicular ferrite and number of oxides under different heat input conditions.

Six ultra-low-carbon high-strength bainitic steel plates were fabricated by controlling rolling and cooling conditions, and effects of bainitic microstructure on tensile and Charpy impact properties were investigated. The microstructural evolution was more critically affected by start cooling temperature and cooling rate than by finish rolling temperature. Bainitic microstructures such as granular bainites (GBs) and bainitic ferrites (BFs) were well developed as the start cooling temperature decreased or the cooling rate increased. When the steels cooled from 973 K or 873 K (700 °C or 600 °C) were compared under the same cooling rate of 10 K/s (10 °C/s), the steels cooled from 973 K (700 °C) consisted mainly of coarse GBs, while the steels cooled from 873 K (600 °C) contained a considerable amount of BFs having high strength, thereby resulting in the higher strength but the lower ductility and upper shelf energy (USE). When the steels cooled from 673 K (400 °C) at a cooling rate of 10 K/s (10 °C/s) or 0.1 K/s (0.1 °C/s) were compared under the same start cooling temperature of 873 K (600 °C), the fast cooled specimens were composed mainly of coarse GBs or BFs, while the slowly cooled specimens were composed mainly of acicular ferrites (AFs). Since AFs had small effective grain size and contained secondary phases finely distributed at grain boundaries, the slowly cooled specimens had a good combination of strength, ductility, and USE, together with very low energy transition temperature (ETT).

A study was made of the effects of carbon, tungsten, molybdenum, and vanadium on the wear resistance and surface roughness of five high-speed steel (HSS) rolls manufactured by the centrifugal casting method. High-temperature wear tests were conducted on these rolls to experimentally simulate the wear process during hot rolling. The HSS rolls contained a large amount (up to 25 vol pct) of carbides, such as MC, M2C, and M7C3 carbides formed in the tempered martensite matrix. The matrix consisted mainly of tempered lath martensite when the carbon content in the matrix was small, and contained a considerable amount of tempered plate martensite when the carbon content increased. The high-temperature wear test results indicated that the wear resistance and surface roughness of the rolls were enhanced when the amount of hard MC carbides formed inside solidification cells increased and their distribution was homogeneous. The best wear resistance and surface roughness were obtained from a roll in which a large amount of MC carbides were homogeneously distributed in the tempered lath martensite matrix. The appropriate contents of the carbon equivalent, tungsten equivalent, and vanadium were 2.0 to 2.3, 9 to 10, and 5 to 6 pct, respectively.

Sticking phenomena occurring during hot rolling of a modified STS 430J1L ferritic stainless steel were investigated in this study by using a pilot-plant-scale rolling machine. As the rolling pass proceeded, the Fe-Cr oxide layer formed in a reheating furnace was destroyed, and the destroyed oxides infiltrated into the rolled steel to form a thin oxide layer in the surface region. The sticking did not occur in the surface region containing oxides, whereas it occurred in the surface region without oxides by the separation of the rolled steel at high temperatures. This indicated that the resistance to sticking increased by the increase in the surface hardness when a considerable amount of oxides were formed in the surface region, and that the sticking could be evaluated by the volume fraction and distribution of oxides formed in the surface region. The lubrication and the increase of the rolling speed and rolling temperature beneficially affected to the resistance to sticking because they accelerated the formation of oxides on the steel surface region. In order to prevent or minimize the sticking, thus, it was suggested to increase the thickness of the oxide layer formed in the reheating furnace and to homogeneously distribute oxides along the surface region by controlling the hot-rolling process.